Integrated circuit chips formed on a substrate are packaged to connect with externally-accessible terminals. The packaging interconnects, system board, and downstream electronics present a load to the chip circuitry. Driver circuitry is used to drive the signals generated on the chip off of the chip while shielding the signal generating circuitry on the chip from the load presented by the connections and further circuits off the chip.
An example of a single-ended driver is shown in FIG. 1. A single input is provided to the gate 101 of a transistor 100. The voltage at the gate 101 is transferred to the output 102. A resistor 103 is coupled between a positive supply voltage 105 and the transistor 100 and ensures the transistor 100 is in the proper operating range. A third terminal of the transistor 100 is tied to ground 106. This simple circuit amplifies the strength of the out-going signal while shielding circuitry attached to the input from the load presented by downstream interconnects and circuitry attached to the output.
As signaling rates increase to 1 Gigabit/second and above, however, maintaining symmetric signals from the single-ended buffer when a signal goes high and when a signal is pulled low, becomes difficult. Asymmetries in these signals exacerbate inter-symbol interference.
Further, a large number of output buffers are provided for devices having a large number of outputs, such as memory devices. The output buffers are tied, directly or indirectly, to positive power supply and ground nodes on the integrated circuit chip. These supplies are, in turn, tied to the package or board supplies through various connections. Accordingly, the positive power supply and ground nodes themselves have some inductance and resistance associated with them. The ground and source voltages, such as nodes 105 and 106 are therefore not ideal. When large numbers of buffers switch, they may actually affect the voltage at nodes 105 and 106, a phenomenon known as ground bounce or simultaneous switching output noise.
Prior systems cope with simultaneous switching output noise through data bus inversion, where the data is inverted at times in an attempt to balance the number of 1s and 0s communicated or through minimizing the number of transitioning signals during the communication. Data bus inversion requires an additional output to indicate when the data is inverted, and when it is not.
Still further, electrical connectors such as wire bonds or conductive traces are used to route signals from a chip to the externally-accessible terminals of a package. Because of the space constraints of the package, these signaling lines can be close enough together to cause cross-talk between the lines.
There is a need for driver circuitry capable of driving a signal off of a chip, but having reduced simultaneous switching output noise and inter-symbol interference. It is also desirable to reduce cross-talk between signaling lines.